Apparatus including secondary emission means for neutralizing an ion beam



Jam 9, 1958 A. BENSUSSAN ETAL 3,363,124

APPARATUS INCLUDING SECONDARY EMISSION MEANS FOR NEUTRALIZING AN ION BEAM 3 Sheets-Sheet 1 Filed April 15, 1964 (sAQS'fU. C31 a] BY ATTORNEY )g1 Jar 9, 1968 A. BENsUssAN ETAL 3,353,124

APPARATUS INCLUDING SECONDARY EMISSION MEANS FOR NEUTRALIZING AN ION BEAM 3 SheetsSheet 2 Filed April 15, 1964 INVENTORS Jan. 9, 1968 A. aENsUssAN ETAL 3,363,124 APPARATUS INCLUDING SECONDARY EMISSION MEANS l FOR NEUTRALIZING AN ION BEAM 3 Sheets-Sheet 5 Filed April l5, 1964 United States Patent Office 3,353,124 Patented Jan. 9, 1968 3,363,124 APPARATUS INCLUDING SECONDARY EMISSION MEANS FOR NEUTRALIZING AN ION BEAM Andr Bensussan, Germaine Vincent, and Robert-Jean Warnecke, Jr., all of 79 Blvd. Haussmann, Paris, France Filed Apr. 15, 1964, Ser. No. 359,960 Claims priority, application France, May 2, 1963, 933,389 12 Claims. (Cl. S13- 63) The present invention relates to ion beam propulsion systems, and more particularly to improvements in neutralized ion beam propulsion devices.

An important problem in the domain of propulsion by ion engines is that of neutralization of the beam. In effect, if the rocket propelled by an ion engine ejected a positive ion beam, the body of the rocket would become charged negatively. The formation within the space of an electric field antagonistic of the accelerator field would result therefrom. It is, therefore, necessary to neutralize the charge of the beam, and it is known to do so by mixing the beam of ions with a beam of electrons which was supplied in the known, prior art devices by a hot cathode.

The presence of such a hot cathode constitutes a complication and a source of diflculties of exploitation, particularly inadmissable, if one envisages the utilization of such neutralized beam in ion motors intended for space vehicles.

The object of the present invention is a propulsion device with a neutralized ion beam in which the presence of a hot cathode is eliminated to achieve neutralization.

The present invention is characterized by the fact that the electrons serving the purpose of neutralization of an ion beam are secondary electrons emitted by a target bombarded by a portion of the ion beam.

According to a first embodiment of the present invention, one takes advantage, for bombarding the target, of the divergence of the beam at the output thereof from the source.

According to a second modified embodiment of the present invention, one causes the beam to converge for bombarding a grid having strong secondary emission properties.

According to a third modification of the present invention, one causes the beam to converge, thereupon lets the beam diverge and takes advantage of this divergence for bombarding by a portion of the beam, a target with strong secondary emission properties.

According to a further feature of the present invention, these aims are attained by ion-optical means, disposed to the rear of the output of the ion beam from the source, ion-optical means which cause the beam to converge after having diverged, in such'a manner as to determine a cross-over point beyond which the beam again diverges in such a manner that the peripheral portion thereof strikes the target whereas the interior portion thereof continues along its path, and the electric field of this interior portion of the beam attracts the secondary electron derived from this target thereby assuring the neutralization of the initial beam.

These ion-optical means may be constituted by a lens which may be either an electrostatic or a magnetic lens.

According to a still further feature of the present invention, one provides adjusting means for controlling the said lenses in such a manner as to displace the cross-over point and therewith to modify the angle of impact of the bombardment ions on the target, thus causing the quantity of secondary electrons emitted to vary and thereby obtaining, at will, a beam either exactly neutralized, over-compensated or under-compensated.

The desired degree of compensation of the beam, obtained with the device of the present invention, will be stable. In effect, a fortuitous reduction of the number of electrons corresponding to the desired degree of neutralization will cause the beam to diverge more strongly, thereby increasing the quantity of secondary electrons emitted, and thus reducing the space charge of the beam. A fortuitous excess of electrons will translate itself, in contrast, by a tightening of the beam, from which follows a diminution of the secondary electrons emitted.

Accordingly, it is an object of the present invention to provide an ion beam propulsion motor utilizing a substantially neutralized ion beam for the propulsion thereof which eliminates, by extremely simple means, the drawbacks encountered by analogous ion beam propulsion motors of the prior art.

Another object of the present invention resides in the provision of an ion beam engine which greatly facilitates the neutralization of the beam and at the same time assures a stable neutralization.

Still another object of the present invention resides in the provision of an ion beam engine which permits the elimination, by simple means involving structurally relative simple and relatively inexpensive parts, of any electric space charge fields that might be antagonistic to the propulsion effect produced by the ion beam.

Still a further object of the present invention resides in the provision of an ion beam propulsion engine dispensing with the need of a hot cathode for neutralizing the ion beam.

A further object of the present invention resides in the provision of an ion beam propulsion engine which relies on the emission of secondary electrons for neutralizing the ion beam.

Another object of the present invention resides in the provision of an ion beam engine provided with control means readily permitting any desired degree of compensation for the neutralization of the beam.

A further object of the present invention resides in the provision of an ion beam engine utilizing a neutralized ion beam in which the neutralization is automatically maintained by extremely simple and reliable means.

These and other objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawing which shows, for purposes of illustration only, several embodiments in accordance with the present invention, and wherein FIGURE 1 is a somewhat schematic transverse cross sectional view through a rst embodiment of an ion propulsion device utilizing a neutralized ion beam, in accordance with the present invention;

FIGURE 2 is a somewhat schematic transverse cross sectional view through a second embodiment of a propulsion device with a neutralized ion beam according to the present invention, comprising a controllable electrostatic lens to permit the adjustment of the neutralization;

FIGURE 3 is a partial transverse cross sectional view 3 through a modified embodiment of the ion beam propulsion device of FIGURE 2;

FIGURE 4 is a partial transverse cross sectional view through still another modied embodiment of the ion beam propulsion device of FIG. 2 in accordance with the present invention, and

FIGURE 4 is a somewhat schematic transverse crosssectional view of a still further modied embodiment of a propulsion device with neutralized ion beam, similar to FIGURE 2, but including a magnetic lens in accordance with the present invention.

Referring now to the drawing wherein like reference numerals are used throughout the various views to designate like parts, and more particularly to FIGURE l, the ion engine represented in this gure comprises an ion source including an enclosure 1 into which a gas such as argon or cesium vapor is admitted through the tubulure 2, and in which is disposed a cathode 3 fed by way of conductors 4 from a conventional source (not shown), and operating as source of ionizing electrons. The ionized gas is concentrated into a beam by the magnetic eld produced by the windings 5 and escapes across the orice 6 pierced into a diaphragm 7. The beam then passes through the orifice 8 provided in the extraction electrode 9, carried by means of source 11) at a negative potential, for example, at -10 kv. with respect to the ion source 1. This electrode 9 comprises preferably a frusto-conical surface 11 on which is disposed a target 48 with strong secondary emission. The electrode 9 is rendered rigid with the source 1 by means of small insulating columns 12, 12.

Operation The device described in'connection with FIGURE 1 operates as follows:

The beam 13 emerging'from the orifice 6 possesses a strong natural divergence after the passage thereof through the orifice 8. Owing to this divergence, a portion of the beam, corresponding to the extreme or outermost path 14 thereof, come to strike the electrode 48, for example, of molybdenum, which emits secondary electrons along the paths 15. These secondary electronsV then mix with the ions whose paths 16 have not struck the electrode 48, and thus form a neutralized beam of which the Vejection exerts a thrust on the missile or rocket propelled by the engine.

Calculations made by the inventors show that with cesium ions, with Va beam intensity of 1 ampere and with an extraction voltage of 10 kv., one obtains with the engine according to the present invention, a thrust of approximately 17 grams.

FIGURE 2 represents a modied embodiment of an ion engine utilizing the same principle of neutralization by secondary electrons emitted under the impact of a portion of the primary ion beam. The engine of FIGURE 2 comprises an ion gun of any desired classical structure, for example, constituted by an enclosure 17 into which a gas such as vaporized cesium is admitted through the tubulure 18 and in which is disposed a cathode 19, fed by way of conductors 20 from a source of conventional nature (not illustrated), and operating as source of ionizing electrons. The ionized gas escapes through the orifice 21 within a diphragm 22, in the form of a beam focused by the winding 23. The beam passes across an extraction electrode 24 of conical shape, fixed on the injector or ejector body 25. This body 25 is insulated from the enclosure 17 by an insulating cylinder 26, and a suitable positive potential is applied from source 27 to the diaphragm 22 with respect to the electrode 24.

According to the present invention, the beam 28 which has diverged at the output of the electrode 24, is subjected to the action of an adjustable lens which, in the illustrated embodiment, is an electrostatic lens constituted by an electrode 29 extending across the body 25 through the insulating passage 30 and carried by means of source -27 at an adjustable intermediate potential between the potentials of the ion source 17 and of the body 25. This lens causes the beam 28 to converge at a cross-over point 31 Whose position varies with the adjustable potential of the electrode 29. According to the present invention, this cross-over point 31 is placed within an electrode 32 of which a portion of the walls, spaced suiriciently far from the electrode 29 in order that the secondary electrons be subjected to the field of the ion beam and not to that of the electrostatic lens, is covered at 33 with a substance having strong secondary emission properties.

Since the beam again begins to diverge after the crossover point 31, it produces the-same phenomena as in FIGURE l, that is, a portion of this beam, corresponding to the extreme or outermost trajectories, strikes the electrode 33 whereas the remaining portion of the beam, corresponding to the trajectories closer to the axis, passes alongside the electrode 33 and mixes with the secondary electrons emitted by this electrode 33 and attracted by the ion space charge. One thus obtains a plasma beam 34 which may be exactly neutralized, over-compensated or under-compensated, depending on the potential of the electrode 29. In effect, the displacement of the crossover point 31 under the effect of the adjustment of this potential produces a modication of the angle of impact on the electrode 33 of the extreme trajectories of the divergent ion beam. A variation, at will, of the quantity of secondary electrons emitted by the electron 33 results therefrom. 1

The beam 34 escapes into the surrounding environment and produces the desired thrust. Orices are provided at 39 and 40 to cause the ion source 17 to communicate with the external vacuum.

FIGURE 3 represents a modified embodiment of the electrode 32 of the device of FIGURE 2. This electrode, designated in FIGURE 3 by the same reference numeral 32, comprises, in a manner analogous tothe electrode 9 of FIGURE 1, a frusto-conical aring 41 along which is disposed the layer 33 with secondary emission. In Ythis figure, as in FIGURE 2, one has shown the cross-over point 31 and the trajectories of both the ions aswell as the secondary electrons which mix to form the neutralized beam 34. With this form of the electrode 32, the angle of impact, which is formed by the extreme or outermost trajectories of the ion beam with the normal or perpendicular to the layer 33, is increased, which increases the secondary emission ratio of this layer 33.

In the embodiment of FIGURE 4, the output of the electrode 32 is obturated by a grid 35 with suitable transparency, realized in a material having a strong coefcient of secondary emission. The transparency may in particular be chosen in such a manner that thefraction of the ion beam captured by this grid produces, due con-V pitch in the direction from the center toward the edges thereof.

The supplementary advantage of the devices accord-V ing to FIGURES 2 and 4 is not to necessitate any magnetic winding for the focusing of the ion beam nor of the secondary electron beam, which diminishes Vthe Weight and cost of the installation.

In contradistinction thereto, the disadvantage of the electrostatic lens is that it does not separate the ions of different masses contained within the gas utilized, Vthat is, isotope ions or multi-atomic ions.

The variation of FIGURE 2 illustrated in FIGURE Y5 eliminates this shortcoming. In th embodiment of FIG- URE 5, one has substituted a magnetic lens forrthe electrostatic lens of FIGURE 2. The drawing only shows the modified portion of FIGURE 2, it being understood that this portion is connected to the same elements of FIG- URE 2 of which the reference numerals are again shown in FIGURE 5.

At the output of the diaphragm 22 of the ion source 17, the beam 2S traverses the extraction electrode 24 which is fixed on the body 42 of the magnetic lens excited by the winding `43. This winding 43 is traversed by a current supplied from a suitable source (not shown) and adjusted by means of the rheostat 44. The vacuum-tight enclosure is delimited by an insulating spacer 45 between the body 42 and the source 17, and by an insulating cylinder 46 on the inside of the body y42.

One gives to the body 42 the shape representing the interior surface of a frusto-conical body in which is disposed the target 48 having strong secondary emission.

The trajectories of the ion beams are designated in FIGURE with the same reference numerals as in FIG- URE 2, and the trajectories of the secondary electrons with the same reference numeral as in FIGURE 1.

It may be readily seen from FIGURE 5 that the beam 2S, y

after having initially diverged, converges and passes through the cross-over point 31, thereupon diverges again in such a manner that the extreme or outermost trajectories thereof strike the electrode 48, whereas the ions following the trajectories closer to the axis, which avoid j the electrode 48, mix with the secondary electrons following the trajectories 15 and forming thus a neutralized beam 34. It will be noted that, for example, when the molecules of the gas employed are polyatomic, the ions formed are either mono-atomic or bi-atomic or possibly tri-atomic, and that in this case, the extreme or outermost trajectories which strike the electrode 48 are followed particularly by the bi-atomic and eventually the tri-atomic ions whereas the mono-atomic ions follow the trajectories which avoid the electrode 48 and constitute a beam to be neutralized. The ions of different masses are thus separated, and the plasma 34 is formed with substantially mono-atomic ions.

The control of current of the magnetic lens permits, in a manner analogous to the control of the Voltage of the electrostatic lens of FIGURE 2, to displace the crossover point 31 and therewith to adjust, at will, the secondary emission ratio of the electrode 48 thereby adjusting the ratio of neutralization of the plasma beam.

While we have shown and` described several embodiments in accordance'with the present invention, it is understood that the same is not limited thereto, but is susceptible of numerous changes and modifications as known to a person skilled in the art, and we therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

We claim: Y

1. In a propulsion engine utilizing the ejection of an ion beam, in combination, comprising:

ion source means,

means for extracting from said source means a diverging ion beam,

means for causing the diverging ion beam to converge thereby to obtain a cross-over point beyond which the beam re-diverges,

means forming a surface of secondary electron emissive material,

and means for bombarding said surface means by a portion of said re-diverging ion beam thereby causing the emission of secondary electrons from said surface,

said surface means being so positioned with respect to said ion beam that the electrons, during movement thereof, mix with said ion beam thereby substantially neutralizing the elctric charge thereof.

2. In a propulsion engine utilizing the ejection of an ion beam, in combination, comprising:

ion source means,

6. means for extracting from said source means a diverging ion beam, means for causing the diverging ion beam to converge thereby to obtain a cross-over point beyond which the beam re-diverges, means forming a surface of secondary electron emissive material, and means for bombarding said surface means by a portion of said re-diverging ion beam thereby causing the emission of secondary electrons from said surface, said surface means being so positioned with respect to said ion beam that the electrons, during movement thereof, mix with said ion beam thereby substantially neutralizing the electric charge thereof, said means forming the emissive surface being provided on the internal walls of a hollow substantially cylindrical piece substantially coaxial with the ion beam and positioned along the path thereof. 3. In a propulsion engine utilizing the ejection of an ion beam, in combination, comprising:

ion source means, means for extracting from said source means a diverging ion beam, means for causing the diverging ion beam to converge thereby to obtain a Cross-over point beyond which the beam re-diverges, means forming a surface of secondary electron emissive material, and means for bombarding said surface means by a portion of said re-diverging ion beam thereby causing the emission of secondary electrons from said surface, said surface means being so positioned with respect to said ion beam that the electrons, during movement thereof, mix with said ion beam thereby substantially neutralizing the electric charge thereof, said means forming the emissive surface being provided on the internal walls of a hollow substantially conical piece forming at least a part of a hollow piece substantially coaxial with said ion beam and positioned along the path thereof. 4. In a propulsion engine utilizing the ejection of an ion beam, in combination, comprising:

ion source means, means for extracting from said source means a diverging ion beam, means for causing the diverging ion beam to converge therebyrto obtain a cross-over point beyond which the beam re-diverges, means forming a surface of secondary electron emissive material, and means for bombarding said surface means by a portion of said re-diverging ion beam thereby causing the emission of secondary electrons from said surface, said surface means being so positioned with respect to said ion beam that the electrons, during movement thereof, mix with said ion beam thereby substantially neutralizing the electric charge thereof, said means forming the emissive surface being constituted by the surfaces of bars of a grid-like structure positioned along the path of thev ion beam in such a manner as to be traversed by at least a substantial portion of the ion beam, 5. In a propulsion engine utilizing the ejection of an ion beam, in combination, comprising:

ion source means, means for extracting from said source means a diverging ion beam, A means for causing the diverging ion beam to converge thereby to obtain a cross-over point beyond which the beam re-diverges, l means forming a surface of secondary electron emissive material,

and means for bombarding said surface means by a portion of said re-diverging ion beam thereby causing the emission of secondary electrons from said surface,

said surface means being so positioned with respect to said ion beam that the electrons, during movement thereof, mix with said ion beam thereby substantially neutralizing the electric charge thereof,

said means forming the emissive surface being constituted by the surfaces of bars of a grid-like structure positioned along the path of the ion beam in such a manner as to be traversed by -at least a subtantial portion of the ion beam,

said grid-like structure having a variable pitch which varies in dependence on the distance from the axis of the beam.

.6. In a propulsion engine utilizing the ejection of an ion beam, in combination, comprising:

ion source means,

means for extracting from said source means a diverging ion beam,

means for causing the diverging ion beam to converge thereby to obtain a cross-over point beyond which the beam re-diverges,

means forming a surface of secondary electron emissive material,

and means for bombarding said surface means by a portion of said re-diverging ion beam thereby causing the emission of secondary electrons from said surface,

said surface means being so positioned with respect to said ion beam that the electrons, during movement thereof, mix with said ion beam thereby substantially neutralizing the electric charge thereof,

said means for causing the ion beam to converge being -an ion-optical lens.

7. In a propulsion engine utilizing the ejection of an ion beam, in combination, comprising:

ion source means,

means for extracting from said source means a diverging ion beam, means for causing the diverging ion beam to converge thereby to obtain a cross-over point'beyond which the beam re-diverges,

means forming a surface of secondary electron emissive material,

and means for bombarding said surface means by a portion of said re-diverging ion beam thereby causing the emission of secondary electrons from said surface,

said surface means being so positioned with respect t said ion beam that the electrons, during movement thereof, mix with said ion beam thereby substantially neutralizing the electric charge thereof,

said means for causing the ion beam to converge being an electrostatic ion-optical lens.

8. In a propulsion engine utilizing the ejection of an ion beam, in combination, comprising:

ion source means,

means for extracting from said source means a diverging ion beam,

means for causing the diverging ion beam to converge thereby to obtain a cross-over point beyond which the beam re-diverges,

means forming a surface of secondary electron emissive material,

and means for bombarding said surface means by a portion of said re-diverging ion beam thereby causing the emission of secondary electrons from said surface,

said surface means being so positioned with respect to said ion beam that the electrons, during movement thereof, mix with said ion beam thereby substantially neutralizing the electric charge thereof,

said means for causing the ion beam to converge being an electrostatic ion-optical lense including means for controlling the voltage applied thereto.

9. In a propulsion engine utilizing the ejection of an ion beam, in combination, comprising:

ion source means,

means for extracting from said source means a diverging ion beam,

means for causing the diverging ion beam to converge thereby to obtain a cross-over point beyond which the beam re-diverges,

means forming a surface of secondary electron emissive material,

and means for bombarding said surface means by a portion of said re-diverging ion beam thereby causing the emission of secondary electrons from said surface,

said surface means being so positioned with respect to said ion beam that the electrons, during movement thereof, mix with said ion beam thereby substantially neutralizing the electric charge thereof, Y

said means for causing said ion beam to converge being a magnetic ion-optical lens.

10. In a propulsion engine utilizing the ejection of anV ion beam, in combination, comprising:

ion source means, means for extracting from said source means a diverging ion beam,

Y means for causing the diverging ion beam to converge thereby to obtain a cross-over point beyond which the beam re-diverges,

means forming a surface of secondary electron emissive material,

and means for bombarding said surface means by a portion of said re-diverging ion beam thereby causing the emission of secondary electrons from said surface,

said surface means being so positioned with respect to said ion beam that the electrons, during movement thereof, mix with said ion beam thereby substantially neutralizing the electric charge thereof,

said means for causing said ion beam to converge being a magnetic ion-optical lens including means for controlling the energizing current of the magnetic lens.

11. In a propulsion engine utilizing the ejection of an ion beam, in combination, comprising:

ion source means, Y

means for extracting from said source means a diverging ion beam,

means for causing the diverging ion beam to converge thereby to obtain a cross-over point beyond which the beam re-diverges,

means forming a surface of secondary electron emissive material,

and means for bombarding said surface means by a portion of said re-diverging ion beam thereby causing the emission of secondary electrons from said surface,

said surface means being so positioned with respect to said ion beam that the electrons, during movement thereof, mix with said ion beam thereby substantially neutralizing the electric charge thereof,

said means for causing said ion beam to converge being a magnetic ion-optical lens including means for controlling the energizing current of the magnetic lens said lens forming part of a body having a substantially conical depression substantially coaxial with said beam, and said means forming the emissive surface being arranged on the Walls of said depression.

12. In a propulsion engine utilizing the ejection of an ion beam, in combination, comprising:

means forming a surface of secondary electron emissive material,

and means for bombarding said surface means by a portion of said diverging ion beam thereby causing emission of secondary electrons from said surface means,

said surface means being so positioned with respect to said ion beam that the electrons thereof move through said ion beam to mix therewith thereby substantially neutralizing the electric charge thereof,

and substantially conical extracting electrode means having an axial aperture substantially coaxial With 10 said ion beam to permit passage therethrough of said beam, said means forming the emissive surface being provided on the conical walls of said extracting electrode means at a place beyond the aperture thereof.

References Cited UNITED STATES PATENTS 3,014,154 12/1961 Ehlers et al 313-63 l0 I AMES W. LAWRENCE, Primary Examiner.

P. DEMEO, Assistant Examiner. 

1. IN A PROPULSION ENGINE UTILIZING THE EJECTIION OF AN ION BEAM, IN COMBINATION, COMPRISING: ION SOURCE MEANS, MEANS FOR EXTRACTING FROM SAID SOURCE MEANS A DIVERGING ION BEAM, MEANS FOR CAUSING THE DIVERGING ION BEAM TO CONVERGE THEREBY TO OBTAIN A CROSS-OVER POINT BEYOUND WHICH THE BEAM RE-DIVERGES, MEANS FORMING A SURFACE OF SECONDARY ELECTRON EMISSIVE MATERIAL, AND MEANS FOR BOMBARDING SAID SURFACE MEANS BY A PORTION OF SAID RE-DIVERGING ION BEAM THEREBY CAUSING THE EMISSION OF SECONDARY ELECTRONS FROM SAID SURFACE, SAID SURFACE MEANS BEING SO POSITIONED WITH RESPECT TO SAID ION BEAM THAT THE ELECTRONS, DURING MOVEMENT THEREOF, MIX WITH SAID ION BEAM THEREBY SUBSTANTIALLY NEUTRALIZING THE ELECTRIC CHARGE THEREOF. 